Unique Formation of a Gauche Conformer in the Crystalline State of 1-Butyl-2,3-dimethylimidazolium Tetrafluoroborate at High Pressures

This study is focused on pressure-induced Raman spectral changes in the ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([C₄dmim][BF₄]) at ∼8 GPa. The sample comprises a methylated C2-H group in the imidazolium cation. As a counterpart of [C₄dmim][BF₄], a sample with an unmethylated C2-H group in the imidazolium cation, i.e., 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄]), was compared with [C₄dmim][BF₄]. Visual observations and Raman spectral changes show crystallization of [C₄dmim][BF₄] at ∼1.3 GPa. However, the unmethylated [C₄mim][BF₄] sample exhibited a pressure-induced glass formation similar to that of most ILs reported in the literature. Therefore, only C2-H-methylation leads to the opposite results for [C₄dmim][BF₄] and [C₄mim][BF₄]. The results of this study also highlight that a gauche conformer forms in the crystalline state of [C₄dmim][BF₄] upon applying pressure. This is unexpected, because trans conformers are typically the major conformers when most imidazolium-based ILs crystallize at high pressures. We analyze this unexpected behavior in terms of the local structural changes in the [C₄dmim] cation using molecular dynamics calculations.